The backing plate provides strength for the fender iron. See the plate on the opposite side panel. Note the iron is broken off on this one on the foreground.

That iron mount is very similar to the touring car fender iron, find one at a swap, and un-rivet it from the touring rear support, the round arm is threaded and fits into the iron mount. On original pickups that arm is a bit longer I think than the touring car, but it should work ok.

A '23-'25 touring rear fender iron mount and threaded round arm pirated off an old touring support was used on this wood panel to hold the rear fender.

I have a drawing of the bracket and will send you a PM when I find it. Also recommend contacting Steve Coniff in Colorado Springs, CO as he has this data. Does the steel box have the fitting to accept the bracket ?

If you don't want to fabricate parts, try to find the '23-'25 touring car rear frame upright that has those irons riveted. Two on are each R or L upright support. One holds the top prop rod, the other holds the fender rod. That iron mount is threaded for the rods, and riveted to the upright.

Usually you can find old rusted uprights and cannibalize these iron mounts.